There is nothing "evil" about installing ceramic tile over an exterior grade plywood in dry areas. However most industry experts will not advocate for or endorse tile installations over plywood in bathrooms or other areas subjected to elevated levels of moisture. Moisture causes wood to expand and contract and these stresses can transfer to the tile causing grout problems as well as loose tiles.
Most of us that has (or had) plywood subloors have experienced some type of water damage on our bathrooms floors at one time or another. A good portion of the emails we receive are plywood problem related. The CTIA have provide a guide concerning where and how plywood substates should be installed:
As far as what andi said about wetting the plywood substrate you should know that this is common. The process is also called "hot mopping" and is a recommended installation practice. Also plywood substrates should be roughed up using sandpaper. Tile should be installed over plywood using a latex modified thinset.
I agree partly with you concerning the construction adhesive. There are a wide variety of adhesives on the market and it is nessesary to choose the right adhesive for the job. We don't suggest it for high footage floors- mostly small areas , e.g. bathrooms where floor height is a critical issue.
The TCA says this, we say that...
1. Construction adhesives are not recommended by the cement or HardiBacker manufacturers.
-Construction adhesives will never be recommended if they are not tested by the manufacturers.
2. Application of construction adhesive to the backerboard is not uniform in coverage and leaves unfilled voids that contribute to increased surface instability and increased deflection problems.
-Construction adhesives (like any product) have detailed instructions regarding their use and proper installation can elliminate these problems. In addition Liquid Nails offers an adhesive (Product #LN-902) that is designed to reduce deflection and to stabilize surfaces.
3. The reason why backerboard manufacturers recommend their product to be installed with thinset mortar is to provide a bed of support. However some of the same manufacturers allow their product to be installed using an organic mastic adhesive which provides little or no bed of support. The only advantage to this type of system seems to be the mechanical bond between the mastic, substrate and backerboard.
-Construction adhesive can not only provide the same bond but unlike mastic, it is waterproof.
Until construction adhesives are properly tested the ceramic tile industry will be slow to warm up to their use. Because backerboard manufacturers won't guarantee their product if they are installed with construction adhesive we advise that their use be limited to small jobs and only as a last resort.
Liquid Nails has a line of adhesives designed for the ceramic tile industry which you may visit at: